The introduction of solids into liquid media requires high mechanical forces. This depends substantially on the wettability of the solid by the surrounding medium and on the affinity with this medium. For the purposes of reducing these dispersing forces it is conventional to use dispersants to facilitate dispersion. These are mostly surfactants or tensides having an anionic, cationic or a non-ionic structure. These agents are directly applied to the solid or added to the dispersing medium in relatively small amounts.
It is further known that these solids tend to flocculate following the dispersion, which nullifies the work earlier done and leads to serious problems. These problems have been accounted for by the London/Van der Waal's forces by which the solids attract each other. For the purposes of counteracting these attractive forces adsorption layers must be applied to the solid. This is done by using such tensides.
During and following the dispersion there is an interaction between the surrounding medium and the solid particle, resulting in a desorption of the tenside by exchange for the surrounding medium present in a higher concentration. This medium, however, is not capable in most cases of building up such stable adsorption layers, resulting in a crash of the whole system. This becomes apparent by the increase in viscosity and pseudoplasticity in liquid systems, losses of gloss and transparency, or opacity in the case of some inorganic pigments and color shifts in lacquers and coatings as well as insufficient color strength development in pigmented synthetics.
To solve this problem, e.g., EP-A 154,678, EP-A 74080, U.S. Pat. No. 4,032,698 and DE-A 24 38 414 propose the use (of) dispersants. These dispersants, however, only lead to a partial solution, particularly with respect to the miscibility without flocculation of different pigments with each other, such as organic pigments and inorganic pigments. Moreover, the pigment pastes prepared by the methods defined tend to interact with the surrounding medium, e.g., after use in lacquers. Consequently, it can be assumed that the adsorption layers built up have insufficient stability against desorption. A number of dispersants proposed in these publications further have the drawback that the storage stability is too poor, which leads to precipitation, phase separation, crystallization, etc. This results in products that are inhomogeneous and useless in practice after a relatively short time.
It is also known that pigments are widely used as colorants, for example, in paints, varnishes, and inks. Such pigments generally have average particle sizes (diameters) in the range of 0.1 to 10 micrometers, more typically, 1 micrometer or greater. To achieve these particle sizes, mechanical devices are most often used to comminute solid particulate into smaller primary particles. The most common mechanical devices include ball mills, attritors, sand/bead mills, and roll mills. The use of mechanical devices merely overcome the forces of attraction of the pigment primary particles, and physically separate these primary particles. Dispersant comes in contact with the particles at this point and is adsorbed thereupon, conveying both electrostatic and steric stability against subsequent flocculation of the particles thus separated. All of these devices require moving parts in order to generate the mechanical forces required to break up the pigment particles. Although milling times may be in the range of several hours, certain pigments require a day or longer in order to separate the particles. Moreover, comminution of the pigment by contact with the milling media, when it occurs, results in pigment surfaces exhibiting a high number of surface asperities (i.e., surface roughness and irregularities). Furthermore, contamination of the dispersions from the mechanical parts of the milling equipment can result due to the intimate contact of the pigment with the milling media. Silicon dioxide, a grinding medium, is a common contaminant found after sand milling, for example.
Another disadvantage of mechanical processing of pigments is the large breadth of distribution of particle sizes resulting from such processes. This results in the presence of particles having diameters of one micrometer or greater, even in dispersions where the average particle size is significantly less. For dispersions requiring transparency in the final article, these larger particles lead to unwanted light scattering and are detrimental. The presence of these micrometer sized particles also leads to inherent instability, or tendency to flocculate, in the dispersions.
More stable pigment dispersions can be obtained by chemically altering the pigment. This often results in smaller average particle diameters but has the disadvantages of requiring a chemical pretreatment of the pigment, still requiring mechanical milling, and still providing a dispersion having a wide particle size distribution.
Current pigment dispersants are effective to some degree in dispersing a pigment at a higher concentration in a non-aqueous or aqueous dispersion medium and in stabilizing the dispersion, but do not offer a satisfactory effect on stabilization of a fine dispersion of the pigment.
The products commonly employed in the prior art i.e., carbon black dispersants in coatings are salts of an acrylic acid copolymer, acetylenic diol surfactants, or polyalcohol ethers which fit into various classes of wetting and dispersing agents (Calbo, Handbook of Coatings Additives, Dekker pg. 516). Such additives could be called on to function as more than a dispersant and can also act in one or more of the following ways: (a) to prevent flocculation, (b) to prevent hard settling, (c) to improve jetness/color/gloss, (d) to control viscosity, and/or (e) to improve wetting of the base resin.
Various considerations are important in determining the usefulness of any additive as a dispersing agent for use with a carbon black or with other pigments, depending upon the product into which such a dispersion is to be incorporated. When used throughout this application the terms pigment(s) or pigment dispersion(s) are intended to encompass various materials which may be intended to impart either color and/or serve some other function, such as for example the use of carbon black in rubber where, in addition to adding color, such also acts as a reinforcing agent.
One of the most important considerations in determining whether a particular dispersant will be useful for use with a given pigment or pigments when such a pigment is to be used in a paint or coating composition is whether such a dispersant/pigment combination will or will not impart a conductive nature or characteristic to the dried paint film or coating into which it has been added.
The automotive industry is replacing and will continue to replace exterior metal body panels on vehicles with plastic and composite body panels. Some reasons for this change are weight reduction, flexibility of design, and lower tooling costs. The replacement of metal body panels by plastics and composites is not without difficulties.
One problem of note is the electrostatic spray painting of plastics. Electrostatic spray painting is the preferred manner of applying automotive topcoats. Spray painting normally gives the best appearance to the vehicle and the electrostatic technique assures the most economical use of the material. The problem arises because plastics do not paint well electrostatically unless a conductive primer is used.
Amongst the most important considerations for determining the utility of any dispersant to be used in conjunction with conductive carbon blacks are the following: the inherent rheological stability of the dispersion, both alone and when added to a formulated paint; maintenance of conductive properties in the resulting coating; resistance to flocculation of the carbon black/dispersant mixture and in the final paint or coating; and ability to achieve low viscosity at high pigment loadings.
The various prior art references of which the applicants are aware which relate to dispersing agents for pigment additives, such as carbon blacks, suffer from a number of shortcomings. The most significant shortcomings of the carbon black dispersants of the prior art, including those used for conductive carbon blacks, are: high levels of dispersant may be required which tends to detrimentally affect the physical properties of formulated paints, such as adversely affecting the resultant humidity resistance, yellowing upon exposure to UV light, loss of cure in melamine cross-linked systems, and other undesirable effects; inability to prevent reflocculation of carbon black, resulting in the loss of electrical conductivity in dried paint films; and incompatibility of the dispersant with the particular resin system selected for use in the final paint formulation.
Additionally, more and more paints are produced which are water-based and completely free from organic solvents, such as glycol ethers. When toning these paints to the desired color, use is made to a great extent of pigment dispersions, which can be used both for water-based paint and for paint based on organic solvents. The pigment dispersions are normally composed of pigments, fillers, dispersing agents and an aqueous phase which contains ethylene glycol, di- and triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol or glycerol. In most cases, the dispersing agent is a nonionic surface-active compound or a combination of nonionic and anionic surfactants. For environmental reasons, it is, however, desirable that the pigment dispersions are solvent-free.
Also, in the production of inks and paints, wetting agents and dispersants facilitate the incorporation of pigments and fillers, which are important formulation constituents that determine significantly the visual appearance and the physicochemical properties of coatings. Optimum utilization requires firstly that the solids are distributed uniformly in paints and inks and secondly that the state of distribution, once attained, is stabilized. In many cases, the stabilizing effect depends on binder components as well. This is particularly the case with acidic (styrene) acrylates, which are used, in particular, in the preparation of printing inks. In these cases, pigment wetting agents are used, whose action consists in wetting the pigment surface rapidly, which displace the air from the surface of the pigments, and replace it by the liquid of the millbase. Especially when solids with a nonpolar surface are used in aqueous coating materials, the wetting must be assisted by wetting agents. This permits favorable development of color strength and thus virtually ideal utilization of the energy introduced.
Moreover, especially in the architectural paints industry, use is made of aqueous pigment pastes, with or without co-solvent, which are used universally for tinting in aqueous emulsion paints on an all-acrylate, styrene/acrylate or silicate basis and in non-polar decorating paints based on long-oil alkyds.
Particularly suitable for this purpose are alkylphenol ethoxylates or fatty alcohol alkoxylates, which also contribute to steric stabilization of dispersed pigment states. The high-performance alkylphenol ethoxylates have come under criticism on eco-toxicological grounds, and their use in laundry detergents and cleaning products is already banned in many countries. Similar bans may be expected for the paint and printing inks industry. Fatty alcohol ethoxylates in many cases fail to achieve the good properties of the alkylphenol-ethoxylates. The nonyl and octyl phenol ethoxylates have shown some utility in enhancing dispersibility of organic pigments. However, they have also demonstrated long term viscosity stability problems. Since they lack groups capable of adsorption, the pigment wetting properties are less pronounced. Moreover, the non-adsorbed portion of this product group, in particular, has the undesirable effect of stabilizing the foam, which can be suppressed only with the aid of substances having a strong defoaming action, which, in turn, induce other adverse phenomena, such as unwanted surface defects. The use of numerous dispersing additives also has a negative impact on the water resistance or light stability of coatings.
The shortcomings of the prior art dispersing agents noted above may be overcome by employing alkoxylates of alicyclic and polycyclic compounds and derivatives thereof in accordance with the present invention.